Phenotype constrains the vocal tract in the most dimorphic mammal, the southern elephant seal (Mirounga leonina)

Abstract

The study of mammal acoustic communication was revolutionized by the application of the source–filter theory, originally developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates a structural link between phenotype and acoustic formants, providing a basis for honest signaling. The phenotype–formant link was validated in many species, but the phenotype–vocal tract link was rarely assessed. We used two dimensional (2D) videogrammetry to estimate the vocal tract length of wild southern elephant seal ( Mirounga leonina (Linnaeus, 1758)) males during their normal vocalization behavior. We showed that ( i) the vocal tract can be measured noninvasively in a large wild mammal; ( ii) the vocal tract depends on the structural phenotype (age, body length, and skull size); ( iii) the nasal tract is more related to the structural phenotype than the buccal tract; and ( iv) the dependence on size, and body length in particular, is stronger than the dependence on age. Altogether, the phenotypic constraint on the vocal tract provides the anatomical basis for honest signaling in elephant seals.